Strain Control Mode Research Articles

Comparation of non-proportional hardening behavior and microscopic mechanism for CP-Ti under strain-controlled and stress-controlled multiaxial low cycle fatigue

Proportional and non-proportional multiaxial low cycle fatigue tests were performed on commercial pure titanium (CP-Ti), employing both strain-controlled and stress-controlled modes. CP-Ti exhibits a four-stage characteristic under higher applied strain amplitude. For other cases and under stress-controlled mode, the initial hardening dissipated, giving way to a typical three-stage cyclic softening characteristics. Two failure modes, fatigue failure and ratcheting failure, occur depending on the applied stress amplitude. CP-Ti exhibits obvious asymmetric strain response under symmetric stress-controlled mode depending on the loading level. CP-Ti demonstrates non-proportional hardening under both strain-controlled mode and stress-controlled mode with different microscopic mechanisms. The additional hardening due to non-proportional loading results in an increase in the axial response stress with increasing multiaxial strain ratio. Interactions between dislocations with different slip systems hinder the development of stable dislocation structures, leading to pronounced non-proportional hardening of CP-Ti under strain-controlled mode. As for stress-controlled mode, the significant growth of twin content improves the grain boundary strength, leading to the non-proportional hardening of CP-Ti.

Cyclic deformation response of annealed low-carbon steel: Insights from ratcheting and LCF experiments

Low cycle fatigue (LCF) and ratcheting experiments were carried out on annealed low-carbon steel at room temperature within a laboratory environment, utilising stress and strain control modes. The annealed low-carbon steel consistently demonstrates a cyclic softening response over its LCF lifespan, across all tested strain amplitudes. Notably, it was observed that ratcheting strain rises while ratcheting life declines with both rising mean stress and stress amplitude. Annealed low-carbon steel, being entirely ferritic and lacking precipitation or substitutional solid solution strengthening or hard phase strengthening, exhibits a restricted ability to withstand or alleviate the accumulation of ratcheting strain, particularly under very low mean stress conditions. In both LCF and ratcheting, significant substructure formation was detected. Nevertheless, there was no discernible difference in substructure formation between LCF and ratcheting when employing electron channelling contrast imaging techniques. The existing mean stress-based fatigue life prediction model has successfully forecasted ratcheting and LCF life within the 102–105 cycles range. A novel approach utilising modulus is introduced to characterise the cyclic hardening/softening behaviour of alloys in stress and strain-controlled experiments. The cyclic hardening model based on modulus effectively captures the responses observed in cyclic hardening/softening during LCF and ratcheting experiments.

An experimental study on the tensile properties of reinforced asphalt pavement

Abstract Utilizing one of the most effective new technologies is the primary goal of this research, which uses a combination of bituminous surfacing materials with geotextile reinforcement sheets, to improve the behavior of asphalt mixes and fatigue reduction. The aim of this research is to analyze the impact of woven geotextile upon fatigue characteristics of hot mix asphalt mixtures. In order to improve the tensile qualities of the asphalt pavement, the experimental program for this study includes the application of geotextile within the wearing layer. Marshall fatigue and stability tests were carried out. Four-point bending test in a strain-controlled mode was adopted in this study. Three micro strain levels (250, 400, 750) at a frequency of 5 Hz were used. The findings demonstrated that unreinforced mixtures have a shorter fatigue life than wearing layers reinforced with geotextile. Additionally, exposure to different temperatures (5, 20, 30°C) showed that the performance of the geotextile interlayer system in cold environments is inferior to that of those in warm and moderate climates. The analyzed geotextile has a positive impact on the performance of the pavement and a large potential to extend the life of the pavement as a whole, according to the obtained results of fatigue life.

Cyclic behavior and damage mechanism of 304 austenitic stainless steel under different control modes

In this work, low cycle fatigue experiments under both strain and stress control modes with different loading levels were conducted on 304 austenitic stainless steel at ambient temperature to discuss the influence of loading mode and level on the cyclic response and physical mechanism. The results demonstrated that the cyclic response generally exhibited three stages: primary cyclic hardening, subsequent cyclic softening, and the secondary hardening finally. The degree of each hardening and softening stage was depended on not only the loading modes but also the strain/stress level. Accelerated cyclic softening and retarded secondary hardening occurred under the stress mode, which made a higher low-cycle fatigue life during stress cyclic loading than that for the strain control mode, especially at relatively high strain/stress level. The secondary hardening stage, occupying the main portion of the lifetime, was proved to be associated with the development of martensitic phase and the extension and intersecting of stacking faults or micro-twin.

Mechanical properties of rock under uniaxial compression tests of different control modes and loading rates

To study the influence of control mode and loading rate on mechanical property of rock, uniaxial compression tests of four types of rocks (gray sandstone, red sandstone, mudstone and granite) are carried out under axial strain control mode and lateral strain control mode respectively. The characteristics of complete stress and strain curves, strength, brittleness and failure modes are analyzed. The results show that control mode has little influence on the pre-peak deformation, stress thresholds, while it has a greater impact on post-peak stress and strain curve, which makes the post-peak deformation stable and controllable, and shows the feature of Class II behavior. With lateral loading rates decrease, post-peak stress and strain curves appear more and more obvious fluctuations in the post-peak stage, and the time required for rock failure increases sharply, but the lateral control rate has little effect on the brittleness of rock. The failure mode of rock samples under axial strain control mode is mainly splitting failure, while that under lateral strain control is gradually changed to shear failure. The smaller the lateral loading control rate is, the more obvious the characteristics of shear failure is. Compared with uniaxial compression tests, under high confining pressure, the lateral dilation deformation is restricted, so peak strength is larger and stress redistribution can be better adjusted and stress fluctuation reduced accordingly in post-peak stage. The research results are an effective supplement to rate-dependent property of rocks and can provide some reference for deformation and strength characteristics research of brittle rock under lateral control mode.

Low-Cycle Fatigue Behavior of Hydrostatic Extruded AZ80 Mg Alloy

The fatigue properties of conventional and hydrostatic extruded AZ80 Mg alloy were fully studied under strain-controlled mode. The hydrostatic extrusion can effectively increase the tensile strength, but decrease the elongation. Consequently, the low cycle fatigue lifespan of the hydrostatic extruded specimens was shorter than that of the conventional extruded ones. The Manson-Coffin-Basquin relationship was adopted to describe the fatigue lifespan of the extruded AZ80 alloys. Finally, the fracture surfaces were observed by scanning electron microscope. The fatigue crack stable propagation zone of the hydrostatic extruded sample has decreased significantly compared with that of the conventional extruded sample. The hydrostatic extruded specimen has small reverse plastic zone size, which leads to microscopically rough fatigue crack propagation zone.

Evaluation of coal mines’ rock mass gas permeability in the equivalent stress zone

Purpose. Based on a comparative analysis of the internal mechanism of shape change of rock samples, which were loaded in specified deformations mode, and geomechanical and gas-dynamic processes in coal mass, to establish a causal link between these phenomena. To qualitatively characterise their gas permeability as a function of the rock’s volume expansion. To justify the possibility of using a full “stress-strain” diagram as a technogenic methane deposit formation model and its spatial localisation. Methodology. Theoretical research is based on using the solid mechanic constitutive principles and results of studying the rock samples failure in the mode of specified strains. Findings. The ability to use a full “stress-strain” diagram for detecting and localising methane reservoirs during the coal seams excavation was proved during the research. It was found that the compaction threshold coincides with the bearing pressure maximum in front of the longwall face. This area corresponds to the rock mass with minimal porosity and minimal filtration, which allows considering it as an envelope of an artificial gas deposit. Regularities that connect the three-dimensional equivalent stress state with the final gas permeability of the gas-saturated coal mass were obtained. These data allow creating a predictive numerical geomechanical model of methane migration paths. Originality. The ability to use a full “stress-strain” diagram in the controlled strain mode for numerical modelling of gas permeability of a methane-saturated coal mass during the mining of coal seams and the determination of technogenic gas deposit boundaries are justified. Dependences of the current and final gas permeability on the rock’s mechanical characteristics in a post-peak strain state are obtained. Practical value. Functional dependencies based on geomechanical models are obtained that allow the identification and localisation of technogenic methane reservoirs in mines during coal seam excavation, with subsequent utilisation of the extracted gas. Furthermore, methane removal enhances mining safety by reducing the risk of gas dynamic phenomena while decreasing gas emissions into the atmosphere contributes to reducing the greenhouse effect.

Low cycle fatigue behaviour of cellular materials: Experimental comparative study of strut-based and gyroid structures made of additively manufactured 316L steel

Cellular materials are an attractive option to improve mechanical properties in lightweight design. Given their complex geometry, cellular materials present small features at the meso-scale that make them highly susceptible to fatigue failures. Fatigue of these materials has been receiving adequate attention only in the last few years, nevertheless, studies of low cycle fatigue (LCF) behaviour are extremely scarce and fragmented. In this study, 316L steel strut-based (FBCCZ) and gyroid cellular specimens were successfully manufactured by laser-powder bed fusion and tested in the LCF regime, considering their post-manufacturing morphological characteristics. The cyclic elastoplastic response revealed a higher stiffness for the strut-based than the gyroid cellular structure. The latter, in contrast, exhibited higher fatigue strength in strain-control mode, thanks to the absence of severe stress and strain raisers when compared to the strut-based counterpart. Overall, strain-life curves of both types of cellular materials are shifted to lower number of cycles to failure with respect to the base material. Detailed fractographic analyses revealed complex and tri-dimensional fracture surfaces for the gyroid specimens, whereas the strut-based lattice displayed planar fracture surfaces.

Studies on high-temperature fatigue behaviour and mechanism for conventionally cast and directed energy deposited forms of Alloy 625

The aim of the present work is to study the low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behaviour of two product forms of Alloy 625. Experiments were performed under total strain control mode using triangular and trapezoidal waveforms under complete reverse loading with R = −1, with a total strain amplitude of ±0.25 %, ±0.4 % and ±0.6 % employing a constant strain rate of 3 × 10−3 s−1 over a temperature range of 298 to 973 K. The additive manufacturing process using the directed energy deposition (DED) technique resulted in an improved fatigue life as compared with the conventional casting (CCA) route. Microstructural analysis using electron back scatter diffraction indicated that an alternating strain got generated within the microstructure and the damage was observed as a strain incompatibility within the partitioned regions of fine and neighbouring coarser grains. The preferred columnar growth occurred in the 〈001〉 direction. Inter-dendritic decohesions were attributed to the Mo, Nb and Ti enrichments. While at ambient temperature, an inter-dendritic de-cohesion and strain incompatibility was the cause for final failure, the occurrence of additional time dependent phenomena such as dynamic strain ageing (DSA), oxidation and creep triggered an earlier fatigue failure at elevated temperatures.

The dependence of linear viscoelasticity limits of cold-recycled mixtures on time of curing and compaction method

Cold-recycled mixtures are currently among the most widely used and investigated methods that enable recycling of old pavement structures in an environmentally friendly manner. Upon milling, the old pavement structure – whose gradation can be improved with addition of virgin aggregate – is mixed and compacted at ambient temperature. The main binding agents are bituminous emulsion and cement. Due to their dual binding behaviour, cold-recycled mixtures present various problems and challenges in terms of correct testing. One of such challenges is the testing of stiffness modulus of cold-recycled mixtures. Apart from dependence of modulus on test temperature and time of curing, recent research proved its highly nonlinear behaviour, much wider than that of classical asphalt mixtures. The paper presents the results of research of linear viscoelasticity limits based on testing conducted in Simple Performance Tester (SPT) device for one cold-recycled mixture recipe, using samples prepared and compacted in different manner. One mixture was prepared in laboratory conditions from materials obtained from the field section. Second mixture was prepared on the basis of field-mixed materials (material prepared and mixed in field, compacted in laboratory). Additionally, cores obtained from the field at 28 and 180 days after compaction were tested. All mixtures were tested at the temperature of 20 °C. Tests were performed 7, 14, 28, 42, 90 and 180 days after compaction. The controlled strain mode was chosen, with strain ranging from 20 to 200 μstrain. Stiffness modulus and phase angle were measured. The test showed that the linear viscoelasticity limits for stiffness modulus testing changed with the time that had elapsed since the compaction for all specimens – the initially low values (of 45–55 μstrain) increased with time and reached constant values (of 75–95 μstrain), with different levels depending on the specimen preparation method.

Large-strain functional fatigue properties of superelastic metastable β titanium and NiTi alloys: A comparative study

This paper investigates the fatigue behavior of superelastic NiTi and two metastable β titanium alloys - commercial Beta III (Ti-11.5Mo-6Zr-4.5Sn wt%) and Ti2448 (Ti–24Nb–4Zr–8Sn wt%) alloys. In situ cyclic tensile tests performed under synchrotron X-ray radiation were used to precisely characterize the stress induced martensitic (SIM) transformation occurring in these alloys. For the NiTi alloy, an intermediate B2-R SIM transformation was detected before the B2-B19′ SIM transformation and no plastic deformation occurred until failure. All metastable β titanium alloys that were solution-treated before testing underwent a reversible β-α" SIM transformation and plastic deformation prior to failure. Low-cycle strain-controlled fatigue tests were performed in tension-tension strain-controlled mode at 37 °C to evaluate large-strain functional fatigue properties. The fatigue life of metastable β titanium alloys was found to be much better than that of NiTi alloy at large applied strains. After a rapid evolution during the first cycles, the mechanical response was found to be constant for NiTi alloy while it evolved continuously for metastable β titanium alloys. In addition, failure occurred suddenly in NiTi, whereas cyclic softening was observed before failure in metastable β titanium alloys. Fatigue properties at higher applied strains are mainly hindered by SIM transformation and defects generated at austenite/martensite interfaces during cycling. This explains why the studied NiTi alloy showed lower fatigue properties than metastable β titanium alloys. In fact, while SIM transformation is homogeneously nucleated in metastable β titanium alloys, SIM transformation is highly localized in NiTi, resulting in higher concentration of defects that promote crack nucleation and, in turn, degrade the functional fatigue properties.

Improved fracture response and electromagnetic interference shielding effectiveness of cementitious composites incorporating pyrolytic bagasse fibers and pine needles

The increasing amount of bagasse and pine needles have become a potential environmental threat to humans and wildlife, as they have serious consequences of catching fire or dumping on land. These wastes have been transformed for the first time into nano carbonaceous inerts via pyrolysis at 500 °C and 700 °C, respectively, in an inert atmosphere and utilized in the preparation of cementitious composites, to improve their strength, fracture response, and electromagnetic interference shielding. The pyrolysis temperature was decided based on thermogravimetric analysis of the selected Agro-wastes. Carbonized material produced by pyrolysis was ground to nano level and characterized through X-ray diffraction analysis, scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman spectroscopy, laser granulometry, and UV–visible spectroscopy. Samples of cement mortar with cement to the sand ratio of 1:1.5 were prepared with the addition of 0.025 %, 0.05 %, 0.08 %, 0.20 %, 0.50 %, and 1.00 % pyrolyzed nano-inerts by wt. of cement after the effective dispersion in water. The uniform dispersion of carbonized particles in cementitious matrix was evidenced through the forensic analysis. Three-point bending test of mortar prisms was conducted at 0.1 mm/min strain rate in strain-controlled mode and the broken halves were tested in compression. A maximum increment of 25.36 % in compressive strength and 15.83 % in flexural strength was achieved due to the addition of 0.20 % pyrolyzed bagasse fibers and 1.00 % pyrolyzed pine needles respectively. A substantial increase of 114.15 % and 101.88 % was achieved in fracture toughness with the addition of 0.20 % carbonized bagasse fibers and 1.00 % carbonized pine needles, respectively. The improvement in strength and ductility is associated with supporting mechanics of crack branching, bridging, and crack contouring verified via SEM micrographs. The inertness of pyrolyzed particles in cementitious composites was verified by X-ray diffraction analysis. Furthermore, the cementitious samples were tested within the frequency range of 8–12 GHz in x-band to check electromagnetic interference shielding effectiveness (EMI-SE), and maximum improvement of 18.02 dB and 20.06 dB was attained with 0.50 % intrusion of pyrolytic bagasse fibers and pine needles, respectively.

Experimental Study on Flexural Properties of Polyurethane–Cement Composites under Temperature Load

Polyurethane cement composite is a new organic–inorganic composite material with high strength, corrosion resistance, and fast curing. It is a complement and alternative to traditional cement materials. The flexural properties of polyurethane cement composites are the basic mechanical index of the material. In order to study the flexural properties under different temperature loads, a molecular model was established, the chemical reaction process of polyurethane cement and the temperature response mechanism was analyzed, and the preparation process of polyurethane cement was proposed. Then, bending tests were carried out in strain-controlled mode to obtain the specimens’ bending strength and stiffness modulus under different temperature loads. The test results showed that the tensile strength of polyurethane cement decreased first, then increased, and finally decreased with the increase in temperature, while the bending stiffness modulus decreased with the increase in temperature. Combined with the theoretical derivation, the damage mode of the samples under different temperature loads was analyzed, and the “L-type” damage strain curve was obtained. The results showed that the proposed theory could effectively explain the mechanism of action and flexural properties of polyurethane cement composites under temperature loading, which is a significant improvement to the application of polyurethane cement composites in practical engineering.

"Ladetes"-A novel device to test deformation behaviors of hydrate-bearing sediments.

Natural gas hydrate (NGH) exploitation is severely restricted by geotechnical problems. Deformation behaviors of the hydrate-bearing strata (HBS) control the occurrence and evolution of geotechnical problems during extracting natural gas from HBS. In this paper, a novel approach named Ladetes is introduced to evaluate the lateral deformation behaviors of the near-wellbore and fracture-filling regions of the HBS. The pressuremeter test and the flat dilatometer test are designed to simulate the inner boundaries of an NGH-producing well and an artificial stimulation fracture for the first time. The device can realize the in situ hydrate formation prior to the experiment and axial loading application throughout the experiment. Both the strain control mode and the stress control mode can be achieved to estimate the deformation characteristics of HBS under different downhole conditions. Prime experiments proved their adaptability and reliability. The Ladetes provides an effective and alternative way of obtaining geotechnical parameters for HBS.

Low cycle fatigue behaviour study of a nano precipitate strengthened ferrite-bainite 780 steel

Low-cycle fatigue behaviour of a newly developed ferrite-bainite 780 steel with the ferrite phase, strengthened by nanosized precipitates was investigated. The LCF tests were conducted in fully reversed strain-controlled mode to determine the cyclic stress response and strain life. The steel showed predominantly cyclic softening behaviour at lower strain amplitudes (up to 0.25%). However, at higher strain amplitudes (≥ 0.35%), cyclic hardening was observed with increase in the number of loading cycles. EBSD study done on the fatigue tested samples revealed grain fragmentation and texture weakening during cyclic deformation. TEM study elucidated the evolution of dislocation structure responsible for cyclic softening/hardening behaviour during cyclic deformation. At lower strain amplitude (0.2%), concurrent presence of random arrangement of dislocations and dislocation tangles were observed with some grains having weakly developed persistent slip bands (PSBs) and veins. At high strain amplitude (0.55%), subgrain structure was found to be the dominant feature along with occurrence of PSBs and veins at few locations. Further to this the role of nano-precipitates in influencing the fatigue behaviour was also investigated.

Validation of time temperature superposition principle for high modulus asphalt concrete in the linear viscoelastic and fatigue domains

AbstractValidation of time–temperature superposition principle (TTSP) in the fatigue domain for a high modulus asphalt concrete (HMAC) is presented in this paper. All tests were performed in tension‐compression under strain control mode. First, TTSP was validated in the linear viscoelastic domain. Then, fatigue tests were performed under three loading conditions, 9.2°C and 5 Hz, 11.0°C and 10 Hz and 12.9°C and 20 Hz, which are equivalent regarding TTSP. Two fatigue protocols were adopted: continuous fatigue test (FT) and fatigue test with rest period (FTRP). For FT, three samples were tested at 180μm/m for each loading condition whereas for FTRP, one sample was tested at 100 μm/m. The data were analysed by comparing the dynamic modulus evolution as a function of time or the fatigue life duration. The results showed that HMAC with fatigue damage remains thermorheologically simple (i.e., respects the TTSP) in the studied temperatures range.

Advanced characterisation of flexural fatigue performance of foamed bitumen stabilised pavement materials

This study investigates the flexural fatigue behaviour of FBS mixtures at different temperatures (11℃, 22℃ and 31℃) bitumen contents (2%, 3% and 4%), secondary binders (cement at 2% and hydrated lime at 2% and 4%), densities (2.14 kg/m3, 2.20 kg/m3 and 2.26 kg/m3) and moisture contents (dry and 90% saturation) with a view to developing a rigorous design approach for FBS pavements. The experimental results showed that the flexural fatigue life of the FBS beams increases with increasing density and bitumen content while it decreases with increasing temperature and moisture content. Further, no noticeable difference was seen in the flexural fatigue life of FBS specimens with 2% and 4% hydrated lime contents while the FBS mixtures with 2% cement showed significantly higher fatigue life than those prepared with 2% hydrated lime. The flexural fatigue life of 3% bitumen and 2% lime FBS mixture under strain-controlled mode was found to be around 21 times higher than that under stress-controlled mode for a given initial flexural strain. The normalised modulus degradation master curve in stress-controlled mode appears to demonstrate a characteristic behaviour of FBS materials under cyclic fatigue loading. Using this new concept and the experimental results, suitable analytical models were developed for the prediction of the flexural fatigue life of FBS mixtures under both stress-controlled and strain-controlled modes taking into account the variations in temperature, bitumen content, moisture content and density of the FBS mixtures.

Analysis of the Low Cycle Fatigue Behavior of DP980 Steel Gas Metal Arc Welded Joints

Dual phase (DP) steels have high strength, while maintaining outstanding elongation capacities. This is possible using a well-controlled thermomechanical process that produces a perfect phase combination in the DP microstructures. However, automotive makers are required to weld the DP steels, which generates a soft zone in the microstructure. In this work, 1.6 mm-thick DP980 steel sheets were welded by gas metal arc welding process to analyze the response of the welded soft zone to cyclic loading conditions. Conducted macrographic and metallography analyses revealed good quality in the appearance of the welded joints, with a complete fusion of the DP980 joint and without the presence of discontinuities. Low cycle fatigue tests of the DP welded joints were conducted under a constant amplitude strain control mode. The welded joints experienced a fatigue life reduction with respect to the DP980 steel of ~16% at strain amplitudes of 0.2, 0.3, and 0.4%. For strain amplitudes larger than 0.6%, the fatigue life of the welded joint was reduced by 39%. Weld thermal cycles combined with metallography analysis indicated that a tempered process of the martensite during the welding was responsible for the soft-zone formation and the poor fatigue response.

Cyclic deformation behaviour of AlSi10Mg aluminium alloy manufactured by laser-beam powder bed fusion

This paper studies the cyclic deformation behaviour of AlSi10Mg aluminium alloy manufactured by laser-beam powder bed fusion for different processing states, namely in the as-built, stress-relieved, and T6 conditions. Low-cycle fatigue tests (Rε = -1) are performed using the single step test method, at room temperature, with a constant strain rate, under strain control mode for strain amplitudes in the range 0.2-1.5%. The main results show that the processing route clearly affects the cyclic stress-strain response of the tested alloy. The strain energy density per cycle of the stress-relived and T6 conditions is much smaller than that of the as-built sate, which is explained by a reduction of the linear portion of hysteresis loops. For lower lives, stress-relieved and T6 conditions led to higher fatigue lives at the same strain amplitude. In addition, on log-log scales, a bi-linear relationship between the plastic strain amplitude and the number of reversals to failure was found for the as-built and stress-relief conditions. On the contrary, for the T6 condition, the data were fitted via a linear function.

Creep-fatigue interaction and damage behavior in 9-12%Cr steel under stress-controlled cycling at elevated temperature: Effects of holding time and loading rate

Considering that most creep-fatigue tests were conducted under strain-controlled mode in previous works, creep-fatigue behavior of 9–12%Cr steel under stress-controlled cycling at elevated temperature was investigated in this work. Particular attention was paid to the effects of holding time and loading rate on the deformation and damage behavior. Results indicated that a remarkable creep ratcheting is generally observed in the creep-fatigue tests with the stress ratio of −1. Ratcheting strain and creep damage progress faster as the holding time becomes larger or the loading rate becomes smaller, resulting in the decrease of the number of cycles to creep-fatigue failure.